This invention describes a method and apparatus for capturing the information in a short pulse. Generally the pulse is electrical or an optical pulse passed through a photo detector. Generally the pulse is the input to a lower bandwidth, electrical domain, analog to digital converter.
Ordinarily, an optical signal is directed to a photo detector and the photo detector output is sent to a track and hold circuit. The track and hold circuit output is the input to an analog to digital converter. After each conversion of the track and hold output, a new sample of the input is captured by the track and hold. In terms of bandwidth, a short input optical pulse requires the track and hold circuit to have a bandwidth inversely proportional to the duration of the input pulse. For a 5 picosecond input pulse, the bandwidth of the track and hold is approximately 200 GHz. Such a broad bandwidth presents its own design issues. In particular a broad bandwidth incurs a substantial penalty in signal to noise ratio.
A continuous time delta-sigma analog to digital converter can efficiently capture the information from a photonic sampler, but such Analog to Digital Converters (ADC) have limited bandwidth. A simple integrating sampler can capture the information from a photonic sampler but is a poor match to the quantizer portion of an ADC because the fraction of the sample period during which the integrator can drive the quantizer is limited by the time required to reset the integrator between samples. Alternatively, a peak detector in place of an integrator could be used but its bandwidth would be on the order of the input pulse duration with the commensurate noise such a bandwidth implies.
There is a need for an interface circuit that will capture the information in the input pulse and preserve it for the track and hold circuit, without the track and hold circuit having a bandwidth dictated by the duration of the pulse.